1. Field of the Invention
The present invention relates to an antenna with long focal length that is compact, robust and can be tested on the ground, mounted on a satellite, and as a non-limiting example, on satellites at a height that is less than the focal length of the antenna.
2. Description of Related Art
Certain spacecraft, and telecommunication satellites in particular, must be furnished with antennae with long focal length (for example more than 3.5 m) which makes it possible to optimize their performance.
By their design, “simple offset” antennae with a large-diameter reflector (for example of at least 2 m) require a considerable focal length (more than or equal to 1.5 times the diameter of the reflector) in order to prevent problems of cross-polarization. The result of this is that, depending on the height of the platform carrying such an antenna, its integration onto this platform may be very difficult, which would make it necessary to choose a complex and heavy solution, and sometimes this integration may be impossible, making it necessary to choose another type of antenna that is heavier and more expensive.
In greater detail, the solutions of the prior art are as follows:                Raising the height of the antenna source: requires a source-carrying structure that is heavy and expensive, and the field of vision of the source interferes with the platform and its appendices. Moreover, the face of the satellite facing the Earth is encumbered by the structure supporting the source, which limits the arrangement of the other antennae of the platform.        Use of a deployment mechanism comprising three axes of which two deployment axes are placed on either side of the arm of the antenna reflector (one on the side of the platform and one on the side of the reflector). In this solution, the arm and the fittings are specific and complex, the reflector turns over (cannot be tested on the ground) during its deployment.        Gregorian antenna: it has considerable weight and cost because it requires two reflectors and dedicated structures for supporting the source and the secondary reflector; the heat control of the source is critical because this source is encased in order to limit the space requirement towards the outside of the satellite, and the face turned towards the Earth is encumbered by the structure carrying the secondary reflector, which limits the arrangement possibilities of the other antennae.        Two-grid antenna: this type of antenna has the advantage of not requiring much focal length—its focal length/diameter ratio is of the order of 1—and it is therefore possible to manage to arrange considerable diameters on platforms of reduced height. However, the space requirement in the stored configuration, due to the height of the peripheral stiffener between its two shells, poses compatibility problems with the nose cones of standard-sized launch vehicles.        